Method for preparing laminated article of cellulosic and polymeric layers

ABSTRACT

A tri-layer laminated article, having utility as a packaging material, is prepared by forming an inner layer of a dispersion of solid polymer particles in molten wax adjacent to and between two outer cellulosic layers. The subsequent application of heat and pressure to the resulting composite causes the wax to impregnate the cellulosic material while the polymer particles form a molten middle layer. Upon cooling the polymer solidifies resulting in an article having two outer cellulosic layers of which at least one is wax impregnated and a middle polymeric layer. Said polymer is insoluble in molten wax at a temperature below the polymer&#39;&#39;s melting point and has a melting point between the melting point of the wax and 300*C.

United States Patent Duling et al. 1451 Feb. 20, 1973 [54] METHOD FORPREPARING 3,373,915 3/1968 Anderson et al. ..161 223 x LAMINATED ARTICLE011 3,553,074 1 1971 Knepp ..161/22o x 3,560,324 2/1971 Ouackenbush..156 309 x CELLULOSIC AND POLYMERIC 3,561,994 2 1971 Dwyre ..l6l/235 xFiled:

LAYERS Inventors: Ir] N. Duling, West Chester; hn C1. Merges, l n. il sboth of Pa.

Assignee: Sun Oil Company, Philadelphia,

2 Aug. 6, 1970 Appl. N0.: 61,719

U.S. c1. ..l56/309, 156/61, 156/285, 156/313, 161 229, 161 235, 264/45,264/261 Int. (:1. ..C05j 7/04 Field 61 Search ..156/169, 228,235, 285,307, 156/309, 311, 326,61; l06/38.25,'230,23l; 264/45-48, 111,261;161/214, 220, 223,

References Cited UNITED STATES PATENTS mmeea 'IIIIIII/IIIIIIA mxxeexxexSECTION "A-A" Primary ExaminerCarl D. Quarforth Assistant ExaminerP. A.Nelson Attorney-George L. Church, Donald R. Johnson and Wilmer E.McCorquodale, Jr.

[57] ABSTRACT A tri-layer laminated article, having utility as apackaging material, is prepared by forming an inner layer of adispersion of solid polymer particles in molten wax adjacent to andbetween two outer cellulosic layers. The subsequent application of heatand pressure to the resulting composite causes the wax to impregnate thecellulosic material while the polymer particles form a molten middlelayer. Upon cooling the polymer solidifies resulting in an articlehaving two outer cellulosic layers of which at least one is waximpregnated and a middle polymeric layer. Said polymer is insoluble inmolten wax at a temperature below the polymers melting point and has amelting point between the melting point of the wax and 300C.

5 Claims, 4 Drawing Figures METHOD FOR PREPARING LAMINATED ARTICLE OFCELLULOSIC AND POLYMERIC LAYERS CROSS REFERENCES TO RELATES APPLICATIONSThe present application is copending with the following applicationfiled same date herewith, Ser. No. 61 ,7l8, E. P. Black, I. N. Duling,J. C. Merges, Jr. and A. F. Talbot, Method for Preparing LaminatedArticle of Metallic, Polymeric and Wax Impregnated Cellulosic Layers.Also, the present application is related to application Ser. No. 884,847filed Dec. 15, 1969 by I. N. Duling and J. C. Merges which describes amethod for preparing a polymer coated, wax impregnat'ed cellulosicsubstrate. All these applications are of common ownership.

BACKGROUND OF THE INVENTION The present invention provides a method forpreparing a tri-layer laminated article having two outer cellulosiclayers and a middle thermoplastic polymeric layer. One of the outercellulosic layers is wax impregnated. The other outer cellulosic layeris or is not wax impregnated depending on when it contacts the polymericmaterial and/or on whether this layer is porous or nonporous. The middlepolymeric layer contributes to the barrier properties of the finallaminated article and is an adhesive. This article has utility as asuperior packaging material.

The aforementioned parent application Ser. No. 884,847 filed Dec. 15,1969 describes a method for wax impregnating porous cellulosic stockwhile being coated with a thermoplastic polymer coating. In the methoddescribed therein, a dispersion of polymer particles in molten wax isapplied to cellulosic stock at a temperature below the melting point ofthe polymer. The'subsequent application of heat, sufficient to melt thepolymer and pressure results in a polymer coated, wax impregnatedcellulosic stock.

In the present application an additional step is incorporated. Theplacing of another cellulosic layer adjacent to the polymer particlesprior to the application of heat and pressure, results in a tri-layerlaminated article of two outer cellulosic layers and an inner polymericlayer. One of the cellulosic layers of said article is wax impregnatedwhereas the other outer layer can be one of the following: a waximpregnated cellulosic layer, a nonporous cellulosic layer or a porouscellulosic layer which is not wax impregnated.

SUMMARY OF THE INVENTION A tri-layer laminated article can be easilyprepared by this invention, said article having one outer waximpregnated cellulosic layer, a middle layer of thermoplastic polymerand the other outer cellulosic layer. The latter, if porous, can beeither wax impregnated or not wax impregnated. Also, the other outercellulosic layer can be nonporous and, therefore, not wax impregnated.In this invention a dispersion of solid polymer particles in molten wax,maintained at a temperature below the melting point of the polymer, isapplied as a coating to a surface of at least one of the cellulosiclayers. The other cellulosic layer, i.e., the layer to which thedispersion was not applied, is brought into contact with the dispersionand then heat and pressure are applied. As a result, substantially allthe molten wax is absorbed by one or both of the cellulosic layerswhereas the polymer particles melt and form a molten middle layer. Uponcooling, the molten polymer solidifies and the tri-layer laminatedarticle is formed.

T he thermoplastic polymer used in the aforementioned invention issubstantially insoluble in molten wax at a temperature below thepolymers melting point and has a melting point between the melting pointof the wax and about 300C.

BRIEF DESCRIPTION OF THE DRAWING FIGS. 1 and 3 schematically illustratetwo methods of forming the tri-layer laminated article in accordancewith the present invention.

FIGS. 2 and 4 are two cross-sectional views of finished laminatedproducts made by the methods described in FIGS. 1 and 3.

DESCRIPTION OF THE INVENTION One method of preparing a tri-layerlaminated article according to this invention is illustrated in FIG. 1.To prepare the dispersion 1, thermoplastic polymer particles and thesolid wax are placed in a suitable container 12. The temperature of thepolymer and wax in the container is raised to above the melting point ofthe wax but below the melting point of the polymer by a suitable heater10. This temperature is maintained throughout the application step.After the wax melts, sufficient agitation is applied by a conventionalmixer 11 to keep the finely divided polymer uniformly distributedthroughout the molten wax. The dispersion, which is a heterogeneouscomposite of thermoplastic polymer in finely divided form and moltenwax, is transferred to a porous cellulosic layer 2 from a container 12.The doctor blade 7 insures that a uniformly thick layer is deposited onthe moving layer 2. When the dispersion 1 contacts the porous cellulosiclayer 2, the absorption of the molten wax begins. Before the secondporous cellulosic layer 3 contacts the polymer particles, sufficienttime elapses so that substantially all the wax in the dispersion 1 isabsorbed by the porous cellulosic layer 2. After the second porouscellulosic layer 3 contacts the polymer particles, rollers 4 and 5 applyheat and pressure to the resulting composite. The heat causes thepolymer particles to melt while the pressure spreads the moltenparticles into one essentially continuous layer. Afterwards the articleis cooled by conventional means such as cold rolls 8 and 9, causing themolten polymer to solidify.-

FIG. 2 is a cross-sectional view of the finished article 6 prepared bythe aforementioned described method. The outer layer 6c represents theporous cellulosic layer 2 now impregnated with wax from the wax polymerdispersion 1. The middle layer 6b is essentially a continuous polymerlayer, the polymer being from the wax-polymer dispersion 1. Becausesubstantially all of the wax is absorbed by the porous cellulosic layerand the polymer is insoluble in the molten wax, the polymeric layer 6bis substantially all polymer. The polymeric layer 6b is continuous inthat it contributes to the total barrier properties of the finishedarticle despite the possible existence of pinholes. Pinholes areextremely small holes that even occasionally appear in extrudedthermoplastic polymer films. The inner polymer layer 6b also binds thetwo outer layers, 6a and 6c, together. The outer layer 6a represents theporous cellulosic layer 3 substantially free of wax or polymer. Whilelayer 6a is substantially free of polymer, sufficient polymer entanglesitself with the surface fibers of the cellulosic layer to provide thedesired bond between the polymeric and cellulosic layers.

Another method of preparing a tri-layer laminated article according tothis invention is illustrated in part in FIG. 3. The dispersion 21,having been prepared as described in the discussion relating to FIG. 1,is transferred to a moving porous cellulosic layer 22. The dispersion21, with the finely divided polymer uniformly distributed through themolten wax and with its temperature maintained below the melting pointof the polymer, is spread in an even thickness by the doctor blade 27across the layer 22. The second cellulosic layer 23, being either porousor nonporous, contacts the dispersion shortly after the dispersion isapplied to layer 22. Thus the cellulosic layer 23, if porous, and layer22 absorb substantially all the wax in the dispersion 21. If cellulosiclayer 23 is nonporous, it absorbs substantially no wax. After the secondlayer 23 contacts the dispersion, rollers 24 and 25 apply heat andpressure to the resulting composite. The heat causes the polymerparticles to melt while the pressure spreads the molten particles intoessentially one continuous layer and forces some of it into thecellulosic layers. Afterwards, the article is cooled by conventionalmeans, such as cold rolls 28 and 29, causing the molten polymer tosolidify.

FIG. 4 is a cross-sectional view of the finished article 26 prepared asdescribed in the discussion relating to FIG. 3. The outer layer 260represents the porous cellulosic layer 22 now impregnated with wax fromthe wax polymer dispersion 21. The inner layer 26b is essentially acontinuous'polymeric layer, the polymer being from the wax-polymerdispersion 21. Because substantially all the wax is absorbed by thecellulosic layers, i.e., 26c and 26a if porous, polymeric layer 26b issubstantially all polymer. The polymeric layer 2611 is continuous inthat it contributes to the total barrier properties of the finishedarticle, despite the possible existence of pinholes. The polymeric layer26b also firmly adheres to the two outer layers, 26a and 260.

In practicing this invention, at least one porous cellulosic layer mustbe able to absorb wax at a reasonable rate. Thus the porous cellulosiclayer which will be impregnated with wax cannot be already coated with asubstance or impregnated with a substance which would substantially slowdown the wax impregnation rate. Typical wax absorption times for coatedand un coated cellulosic materials are shown in the following Table I.

TABLE I Wax Absorption Time of Various Cellulosic Materials Freezercarbon Regular density 10 3 Medium density 18 0.5 Paper plate 20 L5 I!I! II 3 4 Oil can cardboard 26 Yes 6 to 10 28 Yes and 50 to printedFreezer carton Regular density 10 Yes 35 Medium density I8 20 to 22.5Bread wrapper 2.5 12 Glassine paper 133 Time required for a O.l ml. dropof wax to be absorbed into a test sheet at F.

These materials are defined in THE. DICTIONARY OF PAPER, 3rd Edition,American Paper and Pulp Association, 1965.

Kraft liner board, with a wax absorption time of 1 minute as shown inTable I, is a preferred porous cellulosic layer to be impregnated withwax. The medium density freezer carton material, with wax absorptiontime of 20 to 22.5 minutes, is not a preferred porous cellulosic layerto be impregnated with wax.

The difference between porous and nonporous cellulosic material isillustrated by the data shown in Table I. Thus Kraft liner board, with awax absorption time of 1 minute, is an example of a porous cellulosiclayer, whereas, glassine paper with a wax absorption time in excess of100 minutes, i.e., the test is stopped after 100 minutes, is an exampleof nonporous cellulosic material. Glassine paper is an example of apreferred nonporous cellulosic material.

The pulp used to make the various cellulosic layers that can be used inthis invention can be derived from a suitable source such as wood,reclaimed paper, cotton fibers and others fibers such as manila hemp,jute, etc.

The wax used herein can be a petroleum wax obtained by any one of theprocesses described in Chapter 5 of THE CHEMISTRY AND TECHNOLOGY OFWAXES by A. H. Warth, 2nd Edition and can be any one of the refined orunrefined petroleum waxes described in the same chapter. Synthetic waxesthat can be used are described in Chapter 6 of the aforementionedreference.

Petroleum wax is commercially available with a wide range of physicalproperties. Paraffin waxes are available with melting points from about126F. to 153F. (ASTM D87), oil contents from about 0.1 to about 1.2percent (ASTM D721), penetration at 77F. from about 9 to 40 (ASTMD1321), specific gravity at 212F. from about 0.756 to 0.767 (ASTM D287).Microcrystalline waxes are available with melting points from about151F. to 193F. (ASTM D127), oil contents from about 0.4 to about 1.5percent (ASTM D721 and specific gravity at 212F. from about 0.786 to0.795 (ASTM D287). While these different petroleum waxes will beabsorbed at different rates by various cellulosic stocks, our inventioncan be used with any petroleum wax fraction that will be absorbed by thestock.

If a thermoplastic polymer which is soluble in molten wax at atemperature below the melting point of the polymer is used with thisinvention the following problems arise. First if the polymer is solublein the molten wax at a temperature below the melting point of thepolymer, the viscosity of the resulting combination is drasticallyincreased. This resulting high viscosity combination is very differentto handle and to apply to the cellulosic layers. Furthermore, oncontacting the stock the combination is absorbed by the layer. Theremaining combination, after having pressure applied, forms a relativelyweak polymer-wax barrier between the cellulosic layers. Weak means thatthe cellulosic layers can be easily separated from each other throughthe polymer-wax layer. Examples of thermoplastic polymers which aresoluble in molten wax are the lower molecular weight polyethylenes andethylene-vinylacetate copolymers. Thus to avoid the heretofore-mentionedproblems, the thermoplastic polymers used with this invention aresubstantially insoluble in the molten wax at a temperature below themelting point of the polymer.

Examples of thermoplastic polymers which are substantially insoluble inthe molten wax at a temperature below the melting point of the polymerare as follows: polyethylene with a molecular weight in excess of1,000,000, isotactic polypropylene with a molecular weight in excess of100,000, polystyrene, polycarbonate, polymethylmethacrylate, polyvinylchloride, cellulose acetate butyrate, and certain polyamides. Thepolyamides which can be used with this invention are the aliphaticpolyamides such as nylon-3, nylon-4, nylon-6, nylon-7, nylon-8, nylon-9,nylon-l0, nylon-l1, nylon-12, nylon-6,6 nylon-6,10 as well as copolymerssuch as nylon 6,6-6,l0. Also the polyamide can be an aliphatic-aromaticpolyamide such as nylon-6, T. However, all known all-aromaticpolyamides, such as poly(meta-phenylenediamine isophthalamide), havemelting points greater than 300C. (572F.) a temperature at which thediscoloration of cellulosic material occurs and, therefore, should notbe used in the present invention. The methods of preparing thesepolyamides as well as their physical and chemical properties aredefined, in Kirk-Othmer, ENCYCLOPEDIA OF CHEMICAL TECHNOLOGY, Vol. 16,2nd Edition, page 2.

The thermoplastic polymer used in this invention, when added to the wax,is in the form of finely divided particles. 1f the particles are toolarge, say passing through 3 mesh but remaining on 20 mesh, theresulting dispersion is unstable in that the particles settle outrapidly causing problems during the application of the dispersion to thelayer. All references to mesh herein refer to U. S. Sieve Series. Evenif this settling problem can be overcome, the resulting polymer coatingon the stock is of non-uniform thickness and is not continuous; that is,there are some areas where there is no polymer coating. Thus while thisinvention is operable if all the polymer particles pass through a 20mesh, it is preferable that all particles pass through 100 mesh and evenmore preferable that all particles pass through 200 mesh. Particles sizedistribution also influences dispersion stability and dispersionviscosity. At the same wax-polymer ratios a wide range particledistribution, compared to a narrow range particle distribution, tends tohave a greater viscosity and hence greater stability. The limits as toparticle size distribution range can be easily determined by thoseskilled in the art.

The weight ratio of the polymer to the wax used in this inventiondepends on dispersion stability and dispersion viscosity which in turndepends on polymer particle size and particle size distribution.Satisfactory weight ratios are 5 to 45 parts by weight of finely dividedpolymer and 55 to 95 parts by weight of wax; the preferred weight ratiosare 10 to 40 parts by weight of finely divided polymer and to parts byweight of wax.

The preliminary step in practicing this invention is to prepare thedispersion. The solid wax is placed in a suitable container and heateduntil the wax becomes molten; but the resulting temperature should notexceed the melting point of the polymer being used. After the wax melts,the polymer is added; mild agitation is usually necessary to fonn thedispersion. Alternatively, both the solid wax and the polymer particlescan be placed in a container and heated together to a temperature belowthe polymers melting point. Immediately after the dispersion has beenprepared it can be used or it can be cooled and the resulting solid usedat a later time. The permissible temperature range used to prepare thedispersion depends on the melting point of the specific wax being usedand the melting point of the specific polymer being used.

The dispersion, which is a heterogeneous composite of thermoplasticpolymer in finely divided form and molten wax, is applied preferably tothe surface of a porous cellulosic layer. The application of uniformlythick layer of dispersion to the layer can be obtained by using a doctorblade, or by extruding, e.g., curtain coating, or by a roll coater orsome other suitable equipment. The amount of said heterogeneouscomposite being regulated so that substantially all the wax therein, isabsorbable in the pores of the cellulosic material.

After the forming of an inner layer of the dispersion adjacent to andbetween the two cellulosic layers, the inner layer is heated to atemperature above themelt. ing point of the polymer while pressing theouter layers against the inner layer to form an essentially continuousmiddle molten layer. The application of this heat to raise thetemperature of the polymer can precede the application of pressure bysome finite time or can be simultaneous with the pressure. The amount ofpressure applied depends on the type of polymer and ability of the stockto withstand the applied pressure without undesirable deformation.During this step, any wax remaining unabsorbed by the cellulosicmaterial is absorbed into the cellulosic pores.

Subsequently, the article can be allowed to cool or can be cooled by asuitable heat removal device, e.g., cold rollers.

The following examples illustrate this invention:

EXAMPLES To demonstrate that this method could be used with manythermoplastic polymers and different cellulosic stocks, the followingruns were made. Seventy-five parts of petroleum wax were placed in asuitable container and the temperature of the wax was raised to aboveits melting point but below the melting point of the polymer to beadded. To the molten wax 25 parts of the polymer were added. Gentleagitation maintained the polymer particles uniformly distributed in thewax. This dispersion was applied to cellulosic layer in the followingmanner. An oven was maintained at F. In this oven was a roll ofcellulosic material and several inches away was a doctor blade set toapply 2 to 5 mils of dispersion. The material moved from the mountedroll under the doctor blade and out of the oven. The

molten wax-polymer dispersion was applied to the cellulosic layerbetween the roll and the doctor blade. Thus as the layer traveled underthe doctor blade, a predetermined thickness of dispersion was applied.Note that the thickness must be limited to lay down no more waxcomponent than can be absorbed by the cellulosic material. In theseexamples as the cellulosic layer left the oven much of the wax wasabsorbed by the layer because of the slowness at which the layer moved.

Another cellulosic layer was placed on the'coated surface of the firstcellulosic layer. The resulting assembly was placed between the platensof a press. The platen to be brought into immediate contact with thecellulosic layer last added was preheated to between 300F. to 425F.,i.e., a temperature sufficient to fuse the specific polymer. The platenswere closed loosely for a few seconds to heat the cellulosic layer andthe polymer particles. The pressure was then raised to about 500 p.s.i.and maintained for a short time (about sec.). The pressure was releasedand the assembly was removed from the press and cooled to about roomtemperature. The resulting product was a tri-layer laminated articlehaving two outer cellulosic layers, one of which was wax'impregnated,and an essentially continuous middle polymeric layer. The celluloselayers adhered firmly to the polymer as indicated by the peel strengthdata shown in Table II.

TABLE ll Polymers and Temperatures Laminated Article Used to PreparePorous Cellulosic-Polymer-Wax impregnated Cellulosic Layers meltingpoint Fusion Peel Strength Run Polymer of polymer F. lernp. lbs/3" widthl Polyethylene285 310 Tear 2 Polystyrene 250 310 3 Polypropylene 3 30350 4 Polycarbonate350 400 5 Nylon l i376 410 (a) Molecular weight of 1,400,000

(b) Medium impact (c) Molecular weight of 600,000, isotactic (d) Tearrefers to the separation occurring through a cellulosic layer ratherthan the polymeric layer.

(e) Both cellulosic layers were 125 lbs. per ream cupstock Same resultsobtained where both cellulosic layers were 185 lbs. per ream tubestock.

(f) Modified TAPPl-TSOfi-SM46, dry test on 3" wide specimen at rateof2"lmin. and foil being pulled at 180.

board as prepared in the aforementioned manner. With this article a tearseal was obtained.

he petroleum wax used m these examples had the following inspections:

Melting point, ASTM D87 126F. Oil content, ASTM D721 04 Color, Saybolt,ASTM D156 +29 Penetration at 77F., ASTM Dl32l 18 Viscosity, SUS at210F., ASTM D446 38.8 Specific gravity at 212F., ASTM D287 0.760

1. Method of preparing a tri-layer laminated article having two outercellulosic layers and a middle thermoplastic polymeric layer comprising:

a. establishing a heterogeneous composite of thermoplastic polymer infinely divided form and molten wax, said polymer having a melting pointbetween the melting point of the wax and 300C, and being substantiallyinsoluble in molten wax at a temperature below the melting point of thepolymer;

. forming an inner layer of said heterogeneous composite adjacent to andbetween two cellulosic layers, at least one of said cellulosic layersbeing porous and the amount of said heterogeneous composite beinregulated so that substantially all wax therein is absorbable in poresof the cellulosic material and permitting sufficient time to elapse sothat substantially all of the wax in said heterogeneous composite isseparated from said polymer and absorbed into said porous cellulosicmaterial and then;

c. heating the inner layer to a temperature above the melting point ofthe polymer and pressing the outer layers against the inner layer toform an essentially continuous molten layer;

d. and thereafter cooling the molten polymer whereby it solidifiesresulting in said article having at least one cellulosic layerimpregnated with wax.

2. A method according to claim 1 wherein said heterogeneous compositecontains 5 to 45 parts by weight of finely divided polymer and 50 toparts by weight of said wax.

3. A method according to claim 2 wherein all the finely divided polymeris finer than mesh (U. S. Sieve Series).

4. A method according to claim 3 wherein the wax is a petroleum wax.

5. A method according to claim 4 wherein the polymer is selected fromthe following group: polyethylene with a molecular weight in excess of1,000,000, isotactic polypropylene with a molecular weight in excess of100,000, polystyrene, polycarbonate, polymethylmethacrylate, polyvinylchloride, cellulose acetate butyrate, aliphatic polyamide andaliphatic-aromatic polyamide.

* III

1. Method of preparing a tri-layer laminated article having two outercellulosic layers and a middle thermoplastic polymeric layer comprising:a. establishing a heterogeneous composite of thermoplastic polymer infinely divided form and molten wax, said polymer having a melting pointbetween the melting point of the wax and 300*C., and being substantiallyinsoluble in molten wax at a temperature below the melting point of thepolymer; b. forming an inner layer of said heterogeneous compositeadjacent to and between two cellulosic layers, at least one of saidcellulosic layers being porous and the amount of said heterogeneouscomposite bein regulated so that substantially all wax therein isabsorbable in pores of the cellulosic material and permitting sufficienttime to elapse so that substantially all of the wax in saidheterogeneous composite is separated from said polymer and absorbed intosaid porous cellulosic material and then; c. heating the inner layer toa temperature above the melting point of the polymer and pressing theouter layers against the inner layer to form an essentially continuousmolten layer; d. and thereafter cooling the molten polymer whereby itsolidifies resulting in said article having at least one cellulosiclayer impregnated with wax.
 2. A method according to claim 1 whereinsaid heterogeneous composite contains 5 to 45 parts by weight of finelydivided polymer and 50 to 95 parts by weight of said wax.
 3. A methodaccording to claim 2 wherein all the finely divided polymer is finerthan 100 mesh (U. S. Sieve Series).
 4. A method according to claim 3wherein the wax is a petroleum wax.